1. Technical Field
The present invention relates to the field of global navigation satellite systems.
2. Description of the Related Art
As it is known, in order to compute one's position, a GNNS (global navigation satellite system) receiver, for example a GPS (Global Positioning System), receives properly formatted electromagnetic signals transmitted by a constellation of satellites orbiting the earth.
GNNS receivers, which in ordinary operating conditions make use of a Kalman filter procedure in order to compute the user's position, are known.
A Kalman filter computes the current position by means of a weighted linear combination between a first term corresponding to the extrapolation of the position based exclusively on the prior knowledge and a second term which is the position computed just based on the current measures.
At starting-up, when the receiver is turned on, or for any reasons, when the whole past memory of the positioning software is to be reset, the computing procedure usually employed according to conventional methods is the Least Square algorithm. The Least Square algorithm is a one-shot algorithm because it computes the receiver position using a single set of instant measures without any other past information.
Referring to the user's positioning computing by a GNNS receiver, there are environmental conditions which make the positioning particularly difficult and have a high risk of error.
One of the most difficult scenarios is represented by a receiver which, for example, on board a car that has gone through a tunnel and is now coming out of the same. When the receiver is in a tunnel, usually signals from the satellites cannot get inside the tunnel so during this “black out” the receiver cannot compute its position.
On exiting the tunnel, when no measures are available for a short time or a long time, the receiver will supply the user with the new position. According to the known art, the position computing on exiting the tunnel is performed by criteria which do not offer satisfying reliability.